Industrial process fluid pressure transmitters are used to measure the pressure of an industrial process fluid such as a slurry, liquid, vapor or gas in chemical, pulp, petroleum, pharmaceutical, food and/or other fluid processing plants. Industrial process fluid pressure transmitters are often placed near the process fluids, or in field applications. Often these field applications are subject to harsh and varying environmental conditions that provide challenges for designers of such transmitters.
The sensing element in many process fluid pressure transmitters is often a capacitance-based or resistance-based sensor. An isolation diaphragm is generally used to separate the process fluid from the electrically active sensing element thereby preventing the process fluid, which at times can be harsh, corrosive, dirty, contaminated, or at an extremely elevated temperature, from interacting with the electrical components of the pressure transmitter.
Generally, the process fluid acts against the isolation diaphragm generating a deflection of the isolation diaphragm that moves, or otherwise displaces, the fill fluid behind the diaphragm which generates an associated movement of the sensing diaphragm of the pressure sensor. The pressure sensor has an electric characteristic, such as capacitance, or resistance that varies with the applied pressure. The electrical characteristic is measured using measurement circuitry within the process fluid pressure transmitter in order to provide an output signal related to the process fluid pressure. The output signal can further be formatted in accordance with known industrial standard communication protocols and transmitted through a process communication loop to other field devices or a controller.
An in-line process fluid pressure transmitter generally has a single process fluid pressure inlet that can be coupled to a source of process fluid pressure and provides an indication of the process fluid pressure. This indication can be relative to atmosphere, such as a gage indication, or relative to a vacuum, such as an absolute pressure measurement. In-line pressure transmitters that are subject to high maximum working pressure (MWP) present particular design challenges. Simply providing a structure that is able to survive a single application of a maximum working pressure may not be robust enough to survive fatigue with repeated excursions to and beyond the maximum working pressure. Thus, for growing high pressure markets, such as subsea oil and gas wells, it is desirable to provide an in-line process fluid pressure transmitter that is suitable for extended use in such environments.